Skip to main content

Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

See other formats



their nature and the circumstances, and the same holds with glacial acetic

The test of solubility in the ordinary solvents, e.g., in ether, serves to show
if a fatty substance is pure or mixed with extraneous substances insoluble in
that solvent. The test of solubility in alcohol may be used to distinguish castor
oil and fatty acids in general (easily soluble even in the cold) from the majority
of other oils and fats, which are mostly very slightly soluble, especially in the

7. Acid Number
(Acidity Index)

The acid number is defined as the number of milligrams of potassium
hydroxide (KOH) necessary to neutralise the free fatty acids in a gram of
the fatty substance. From this number the amount of free acids contained
in a fat may hence be deduced. The determination is made as follows :

An exact weight of about 5 grams of the fat is heated in a flat-bottomed
flask on a water-bath with 50-60 c.c. of 96% alcohol,1 and kept well shaken
until the alcohol begins to boil. Seven or eight drops of phenolphthalein
solution (i% in 95% alcohol) are then added and the liquid titrated with
decinormal potassium hydroxide solution until a persistent red colour
appears. If solid fat separates during the titration, the flask is placed on
the water-bath until the fat melts. The volume of potash solution used
gives the acid number (i c.c. N/io-KOH = 0-00561 gram KOH).

From the acid number thus obtained the percentage of free acids in
a fat may be calculated ; this is usually expressed as oleic acid, the mole-
cular weight of which is 282 (corresponding with 56-1 of KOH). The calcu-
lation is made by the formula,

n X 0-0282
y, = .------------ x 100,


where n is the number of c.c. of N/io-KOH used, p the weight of the sub-
stance and * the percentage of oleic acid in the substance.

The free acids in a fat are sometimes expressed as sulphuric anhydride
(S03), the formula then becoming

; ,                                      _ n x 0-004


x in this case being the percentage of S08.

EXAMPLE : For 5-223 grams of fat, 4-2 c.c. of N/io-KOH were used,
bince i c.c. of the potash corresponds with 0-00561 gram of KOH, i.e with 5-61
milligrams, the acid number will be


5-61 X 4-2

c c       th. ,l
4 vo?s   of ether o

= 4-51-

neutral to Phenolphthalein by means of

i                   °nly Sllghtly S0luble in alcoho1 OT' ™re especi-

Col°"red f lution in ^ohol, 100-150 c.c. of the latter must
°f substance:   e-g- * grams are treated with 50-100
a mixture of <'> ' vo1' of ^hol ander and the acids washed with hot water